1. Field of the Invention
The present invention relates to magneto-resistive elements used in reading heads of magnetic recording devices such as for optomagnetic disks, hard disks, digital data streamers (DDS), or digital VCRs, in magnetic sensors for detecting rotation speed, and in magnetic random access memory (MRAM).
2. Description of the Related Art
CPP (current perpendicular to the plane)-GMR elements are magnetoresistive elements using spin-dependent scattering between ferromagnetic layers sandwiching a conductive intermediate layer, whereas TMR elements are magnetoresistive elements using spin tunneling between ferromagnetic layers sandwiching an extremely thin insulating intermediate layer. In both elements, the current flows perpendicular to the film surfaces of the multilayer structure. In these elements, to increase the reproducibility of changes of the magnetization displacement angle, one of the ferromagnetic layers may be devised as a pinned magnetic layer on which an antiferromagnetic layer such as FeMn or IrMn is layered. Also, if a layered ferrimagnetic structure including antiferromagnetic coupling, for example Co/Ru/Co, is layered on the antiferromagnetic layer, then the pinning magnetic field of the pinned magnetic layer can be increased even further.
Half-metals in which the spin polarization is expected to be 100% by band calculation have garnered attention as ferromagnetic materials. In particular for TMR elements, the magnetic resistance change ratio is higher, the higher the spin polarization of the ferromagnetic material is.
A high thermal resistance is required when the magneto-resistive element is applied to magnetic heads, MRAM memory elements or the like. For example, if the TMR element is used for an MRAM, a thermal process at about 400xc2x0 C. is performed in a semiconductor process of hydrogen sintering or a passivation process.
However, when an element having an antiferromagnetic layer is heated to at least 300xc2x0 C., the spin polarization of the magnetic layers decreases due to diffusion of the Mn included in the antiferromagnetic layer, and the pinning magnetic field is decreased due to the change of the composition of the antiferromagnetic layer (see S. Cardoso et.al., J. Appl. Phys. 87, 6058(2000)). Also, in elements in which a layered ferrimagnetic structure is layered on an antiferromagnetic material, the layer structure of the layered ferrimagnetic structure is disturbed during thermal processing, so that an increase of the pinning magnetic field cannot be expected. Thus, an improvement of the thermal resistance is desired for magneto-resistive elements. An increase in the thermal resistance also is desired for CIP (current in plane)xe2x80x94GMR elements, in which the current flows in the film plane.
Furthermore, a high magnetic resistance change ratio still has not been attained at room temperatures with elements using half metals. In particular when forming an oxide half-metal material by sputtering with an oxide target, the oxygen amount easily deviates from the stoichiometric ratio, and it is difficult to obtain high-quality magnetic thin films. But there is a possibility that higher magnetic resistance change ratios can be obtained with magneto-resistive elements using half metals.
Furthermore, in particular in TMR elements, there is the problem that there are sometimes large non-symmetries in the response to external magnetic fields.
According to a first aspect of the present invention, a magnetoresistive element includes an intermediate layer and a pair of magnetic layers sandwiching the intermediate layer, and one of the magnetic layers is a pinned magnetic layer in which magnetization rotation with respect to an external magnetic field is harder than in the other magnetic layer. The pinned magnetic layer includes at least one non-magnetic film and magnetic films sandwiching the non-magnetic films, and the magnetic films are magnetostatically coupled to one another via the non-magnetic film.
The magnetic films are magnetized antiparallel to one another with the non-magnetic film arranged between them, and the magnetostatic energy forms a closed magnetic circuit, that is, the magnetic films are magnetostatically coupled, so that leaking magnetic fields causing magnetic field shifts in the other magnetic layer (free magnetic layer) are suppressed. Also in layered ferrimagnetic structures utilizing antiferromagnetic coupling that have been used conventionally, the magnetization directions become antiparallel. However, layered ferrimagnetic structures utilize the RKKY effect (Ruderman-Kittel-Kasuya-Yoshida effect), so that they are very sensitive to the thickness of the non-magnetic film. By contrast, when using magnetostatic coupling, the dependency on the thickness is relatively small. Furthermore, when magnetostatic coupling is used, the non-magnetic film itself can be thick. Thus, the thermal stability of the element can be improved by using magnetostatic coupling.
According to a second aspect of the present invention, a magnetoresistive element includes an intermediate layer and a pair of magnetic layers sandwiching the intermediate layer, and one of the magnetic layers is a pinned magnetic layer in which magnetization rotation with respect to an external magnetic field is harder than in the other magnetic layer. The pinned magnetic layer includes at least one non-magnetic film and magnetic films sandwiching the non-magnetic film, and the magnetic films are coupled to one another by magnetostatic or antiferromagnetic coupling via the non-magnetic film, and when the magnetic films are magnetic films that are arranged at positions m (with m being an integer of 1 or greater) from the intermediate layer, Mm is an average saturation magnetization of the magnetic films m and dm is their respective average film thickness, Mdo is the sum of the products Mmxc3x97dm of the magnetic films with odd m and Mde is the sum of the products Mmxc3x97dm of the magnetic films with even m, then
0.5 less than Mde/Mdo less than 1.
In this element, the magnetic films are magnetized antiparallel by antiferromagnetic or magnetostatic coupling, with non-magnetic films disposed between them. To completely eradicate the magnetic field leaking from the pinned magnetic layer, Mde/Mdo should be set to 1. However, as the result of experiments explained below, it was found that in particular in TMR elements, positive magnetic coupling occurs between the pinned magnetic layer and the free magnetic layer. This coupling causes non-symmetries in the response of the magnetic resistance to external magnetic fields. In these elements, it is more advantageous to set Mde/Mdo less than 1, so that a leaking magnetic field canceling the positive magnetic coupling is generated (causing negative magnetic coupling), improving non-symmetries. When the leaking magnetic field is too large, non-symmetries occur on the negative coupling side, so that it is preferable to set Mde/Mdoxe2x89xa70.6.
According to a third aspect of the present invention, a magnetoresistive element includes an intermediate layer and a pair of magnetic layers sandwiching the intermediate layer. At least one of the magnetic layers includes an oxide ferrite having a plane orientation with a (100), (110) or (111) plane, and a change in magnetic resistance is detected by introducing an external magnetic field in the plane. The external magnetic field is preferably introduced in a direction of the axis of easy magnetization in the plane but the oxide ferrite can be non-orientated in the plane.
Examples of oxide ferrites include MnZn ferrite, NiZn ferrite and magnetite (Fe3O4). When grown in an orientated state, oxide ferrites have a relatively high magnetic resistance change ratio in the (100), (110) or (111) plane. And when grown epitaxially, the magnetization responsiveness of the magnetic resistance changes with respect to external magnetic fields is increased by introducing an external magnetic field in the direction of the axis of easy magnetization.
Yet another aspect of the present invention provides a method that is suitable for manufacturing the elements as described above. This method is suitable for manufacturing a magnetoresistive element including an intermediate layer and a pair of magnetic layers sandwiching the intermediate layer, and at least one of the magnetic layers includes an oxide ferrite. The method includes forming the oxide ferrite by sputtering with an oxide target while applying a bias voltage to a substrate including a plane on which the oxide ferrite is to be formed so as to adjust an amount of oxygen supplied to the oxide ferrite from the oxide target.
When sputtering with oxide targets, tiny composition deviations easily deteriorate the properties of the element. With the above-described method, the composition control becomes easier, so that the reproducibility of the element increases. This method is also suitable for other compound magnetic thin films. That is to say, according to yet another aspect of the present invention, a method for forming a magnetic compound film is provided, which includes forming the magnetic compound film by sputtering with a compound target while applying a bias voltage to a substrate including a plane on which the magnetic compound film is to be formed so as to adjust the amount of at least one selected from oxygen and nitrogen supplied to the magnetic compound film from the compound target. With this method, it is possible to obtain compound magnetic thin films of the desired stoichoimetric ratio with high reproducibility.
The present invention includes the element that can be described from two or more of the aspects. The element of the present invention can include more layers, for example, two or more non-magnetic layers and magnetic layers sandwiching the non-magnetic layers.